INTRODUCTION Repeated contact with certain petroleum oils is known to have produced cancer of the skin of man under certain occupational conditions. The best documented experiences have been the occurrences among operators of spinning mules in the British cotton industry and among wax pressmen in two refineries of the American petroleum industry (1@2@3). In seeking the causative agent@ investigators in various countries have gathered evidence of the existence in these oils of polycyclic aromatic hydrocarbons that are capable of inducing cancer of the skin of mice when applied there on under suitable experimental conditions. During World War II@ it became apparent that there was a trend in the composition of certain products of modern refining processes (e.g.@ catalytic cracking) in the petroleum industry towards higher concentrations of such aromatic carcinogens. Experimentally@ it was found that some catalytically cracked oils were capable of inducing cancer of the skin of mice with considerable speed and uniformity. Though lacking confirmation in terms of practical experience in the petroleum industry@ this fact came to be regarded as the justification for a comprehensive investigation of the cancer-inducing potentialities of a large number of products and processes. Such an investigation was initiated in 1946 in The Kettering Laboratory@ in the Department of Preventive Medicine and Industrial Health of the University of Cincinnati@ under the financial sponsorship of the American Petroleum Institute.